In patents GB1181794, U.S. Pat. Nos. 3,518,050 and 3,645,910, and published patent application US20030069132A1, Woodhead describes the formation of a zirconium sol. The processes described in these documents do not include a complexing agent as defined in the present invention and thus they result in insufficient structure generation (polymerisation extent) in the sol for use in subsequent applications. The oxide products referred to in the Woodhead patents are described as very dense, and, for example, would be completely unsuitable for catalytic applications.
U.S. Pat. No. 3,776,987 relates to various metal sols, including a zirconia sol as prepared in U.S. Pat. No. 3,518,050, to which are added an organic polymer (i.e. containing hydroxy, carboxyl and amino groups). However, no heating step takes place in the presence of the organic and it plays no part in controlling the thermal hydrolysis of the Zr-precursor in the sol-forming stage. In addition, the sol has a pH of around 9-10. Similarly, U.S. Pat. No. 5,750,459 uses a zirconium precursor prepared by dissolving zirconium basic carbonate in nitric acid. A thickening agent is then added prior to dripping into an alkaline gelation bath. There is no use of a complexing agent as defined in the present invention.
CN102040379, CN102775143 and CN102464353 all relate to the formation of zirconia colloids/sols. However, none of them disclose the use of a complexing agent as defined in the present invention in the formation of the zirconium sol.
U.S. Pat. No. 4,788,045 mentions the use of a zirconium oxynitrate precursor heated at 150° C./24 hrs. U.S. Pat. No. 3,359,213 discusses the formation of colloidal hydrous oxides of zirconia. US20060115397 mentions the use of a zirconia sol prepared by the dissolution of zirconium basic carbonate in nitric acid. None of these documents mentions the use of a complexing agent as defined in the present invention.
EP1994979A1 refers to the formation of a zirconia sol by dissolution of a zirconium compound (e.g. zirconium basic carbonate) into an aqueous solution of a dicarboxylic acid (e.g. oxalic acid). The final product typically has a very low concentration (˜2% ZrO2) and a pH of ˜6-7, which is significantly different from the composition of the invention. In addition, the process does not involve the dissolution of a zirconium salt in nitric, acetic or hydrochloric acid.
U.S. Pat. No. 5,466,846 relates to the preparation of aqueous solutions of zirconium chelates (α-hydroxycarboxylic acids) with specific mention of mandelic acid, e.g. sodium zirconium mandelate, potassium zirconium mandelate and amine zirconium mandelate. The [α-hydroxycarboxylate]:Zr ratio is from 0.5:1 to 20:1, so significantly higher than the amount of complexing agent used in the present invention.
U.S. Pat. No. 5,234,870 relates to the formation of zirconia sols by heat treatment (hydrolysis) of ammonium zirconium carbonate in the presence of organic chelating agents (e.g. oxyacids). The use of ammonium zirconium carbonate results in the sol having a pH of >7, which is significantly higher than that of the present invention.
An improved process for preparing zirconium sols/solutions, in which the sol/solution can impart advantageous properties in products formed therefrom, has been sought.